This invention is related in general to the field of optical devices. More particularly, the invention is related to apparatus and method of controlling a gated power supply in an image intensifier with a micro-channel plate.
There are many industrial and military applications of image intensifiers for sensing and amplifying radiation images of low intensity. Typical image intensifier devices employ micro-channel plates for multiplying an electron image corresponding to the sensed radiation image. More specifically, such devices include a photocathode for converting the incident radiation image into a corresponding electron image, which is multiplied by the micro-channel plate (MCP). The multiplied electron image is directed onto a phosphorous screen for providing an intensified display of the sensed radiation image. The gain is achieved by the micro-channel plate, which typically includes many channels or passages, each presenting secondary electron emissive surfaces. As the electrons of the image are directed through the channels, they repeatedly collide with the secondary emissive surfaces and their numbers multiply as a result.
Image intensifier tubes employing micro-channel plates are subject to saturation in the displayed output image when the intensity of the output image exceeds its operating range. Operation of the micro-channel plate image intensifier in the saturated region causes the gain within the bright areas of the displayed image to be significantly lower than that of the dark areas, thereby destroying image contrast. Additionally, saturation also ruins the focus and causes image distortion. Gating the power supply is a conventional solution used to vary the effective duty cycle and therefore the gain of the image intensifier to avoid saturation.
However, image intensifier devices using gated power supplies suffer from a flickering or strobing effect. When a periodically modulated ambient light source, such as fluorescent light, is present, an undesirable beat frequency results if the power supply is gated at a multiple or sub-multiple of the light frequency.
Accordingly, there is a need for preventing the undesirable flickering effect caused by an ambient periodically modulated light interfering with the gated power supply frequency. In accordance with the present invention, apparatus and a method are provided which eliminates or substantially reduces the disadvantages associated with prior circuits.
In one aspect of the invention, apparatus for controlling a gated power supply of an image intensifier having a micro-channel plate includes a current-to-voltage converter coupled to the micro-channel plate operable to convert a strobing current on the micro-channel plate to a strobing voltage signal, a AC-to-DC converter coupled to the current-to-voltage converter operable to convert the flickering voltage signal to a voltage level signal, and a control circuit coupled to the AC-to-DC converter operable to generate a varying frequency gating control signal in response to the frequency of the strobing current.
In another aspect of the invention, apparatus for controlling a gated power supply of an image intensifier having a micro-channel plate includes a current-to-voltage converter coupled to the micro-channel plate operable to convert a strobing current on the micro-channel plate to a strobing voltage signal, a AC-to-DC converter coupled to the current-to-voltage converter operable to convert the flickering voltage signal to a voltage level signal, and an oscillator coupled to the AC-to-DC converter operable to generate a gating control signal in response to the voltage level signal, the gating control signal changing the gated power supply frequency to substantially minimize the strobing current.
In yet another aspect of the invention, a method of gating a power supply of a photocathode in an image intensifier having a micro-channel plate, includes the steps of sensing a strobing current in the micro-channel plate, and gradually varying the gating frequency of the gated power supply until the strobing current is substantially eliminated.
The present invention is operable to minimize or substantially eliminate the undesirable flickering caused by the beat frequency created between the gated power supply frequency and a periodically modulated ambient light, such as fluorescent light or the light from a computer monitor. The present invention, upon detection of the flickering, gradually changes the photocathode power supply frequency until the flickering is eliminated.